There is a growing demand for DC-to-DC converters with improved conversion efficiency and reduced size. Design techniques vary, with some techniques reducing the voltage or current, others increasing the voltage or current, and still others alternately increasing or decreasing voltage or current. In a typical DC-to-DC converter, one or more switches connect to input power, a capacitor or inductor stores excess energy in one phase, and in another phase the stored energy is released to output nodes.
Each technique has its own advantages and disadvantages. There are multiple tradeoffs between component sizes, overall efficiency and optimal zones of input to output voltage ratios. For example, increasing switching frequency usually results in a reduction of component size, but concomitantly increase switching losses due to parasitic capacitances and switch transition losses. As a result overall efficiency is reduced.
A common method of AC-to-DC conversion is to use a Bridge Rectifier to convert AC input to DC, and then use a DC-to-DC switching converter to generate the proper DC output. Past attempts to simultaneously meet the objectives of improving conversion efficiency and reducing the converter size have been dominated by overall efficiency concerns. Efficiency is particularly important over a wider input to output voltage ratio, as experienced in AC-to-DC conversions. An AC source usually goes through a wide range of momentary voltages in each cycle, hindering any attempt to optimize a DC-to-DC converter for a specific input to output voltage ratio.
One approach to overcome the adverse effect of big variations in input voltage is to store charge in a storage capacitor, and to use the stored charge at the times the AC input voltage is below a certain level, referred to as a “transition period”. There are two disadvantages associated with this method. First, a relatively large capacitor is required to store enough energy through a transition period. Second, during transition periods small or no current is drained from the input, which results in electrical current drain from the input AC line to transpire over a narrower time frame, mostly around peak input voltages. This eventually results in inefficient power transfer and a lower Power Factor.
Switching converters can be classified into three major converter classes based upon the number of active power switches employed. The two-switch converter class includes buck, boost and flyback converters. The three-switch converter class includes forward converters. The four-switch converter class includes half-bridge and push-pull switching DC-to-DC converters. The switches may be active or passive. An active switch is controlled by modulating a gate. A passive switch, such as a diode, does not require a separate control.
In view of the foregoing, it would be desirable to provide improved techniques for power conversion. In particular, it would be desirable to provide reduced switching losses and reduced component sizes in a converter utilized in connection with wide voltage swings.